1. Field of the Invention
This invention relates to compositions comprising ethylene/vinyl acetate copolymers, tackifying resins and optionally polyolefins (for example, polyethylene and polypropylene) that provide strong, tight hermetic heat seals with an easily peelable opening feature to fluoropolymers such as polychlorotrifluoroethylene. These compositions will be useful in extrusion coating, cast and blown film processes as well as lamination to other substrates.
This invention also relates to multilayer structures and packages comprising these compositions.
2. Description of the Related Art
The packaging industry uses a wide variety of films and containers prepared from various thermoplastic resins and compositions for packaging food and non-food products. These packages must provide adequate protection (for example, protection from mechanical damage, barriers to air or moisture, etc.) of the product contained within until the consumer is ready to use the product. It is also desirable for the package to be designed to allow the consumer easy access to the product at the appropriate time.
Materials used in packaging include polychlorotrifluoroethylene (PCTFE) fluoropolymers. A particular advantage of PCTFE is its high moisture and oxygen barrier properties, allowing greater shelf life of the product packaged. A number of homopolymers and copolymers derived from polymerization of chlorotrifluoroethylene are available commercially under the tradename Aclar®) from Honeywell, Inc., Morristown, N.J. It is generally difficult to find compositions that provide good adhesion to fluoropolymers such as PCTFE because of their inherent properties.
Typically a PCTFE layer is adhered directly to a second layer of polyvinyl chloride (PVC) or polyethylene terephthalate (PET) to create a structure that can be formed into packages. PVC or PET is typically used as the sealing surface as PVC and PET have high affinity to many heat sealable materials. PVC or PET also provides bulk to the structure, as they are far less expensive than the PCTFE. Both materials provide high clarity and formability. However, PVC and PET have insufficient oxygen and moisture barrier properties when used as the seal surface. When the barrier top sealant web is heat sealed to the PVC or PET, the barrier properties of the PCTFE cannot be fully realized. Oxygen and moisture permeate through the PVC or PET between the heat-sealable lidding material and the PCTFE, leading to shortened shelf life.
Existing sealant systems that bond to PCTFE include solvent-borne systems based on, among others, styrene butadiene or amorphous polyester. The sealants are applied by roller or gravure systems to carrier webs (e.g. 1 mil-thick foil) and require drying at elevated temperature to remove the solvents and maintain low amounts of residual solvents. The heat-seal coatings produced in this manner are generally very thin and are limited to ingredients that are soluble in solvent systems.